Cuny students are about to get a tough lesson in economics — tuition is expected to soar 31 percent over four years.

The City University’s Board of Trustees is voting today on hikes of $300 a year — including one that already went into effect for 2011-12.

By 2015 under the plan, the cost for undergrads would be $6,330 a year, compared with $4,830 last year.

The fee for community- college students would shoot up to $4,800 by 2015.

The controversial hikes were approved by state lawmakers this summer as part of a “rational tuition” policy that CUNY officials say will eliminate the kind of sudden increases enacted in past years.

But students, some of whom clashed with campus cops at a public hearing on the tuition hikes last week at Baruch College, railed against the CUNY for seeking to balance its budget on their backs.

“I am unequivocally opposed to any tuition increase for public higher education,” said University Student Senate Chair Kafui Kouakou.

Today’s meeting is also being held at Baruch, where classes scheduled for after 3 p.m. have been canceled.

Board of Trustees documents show that the university has been strained by an increase in enrollment of about 9.6 percent since 2007, combined with a $300 million reduction in its budget.

The documents stress that 59 percent of full-time CUNY students had their tuition fully covered last year by a combination of state and federal programs.

“In order to maintain and further strengthen academic quality and student support services, the recommended increases are necessary,” the documents say.

The trustees will have to approve the annual hikes each year.

The boisterous battle at CUNY stands in stark contrast to last week’s relatively unchallenged approval by the State University of New York’s board of similar tuition hikes — which had the backing of the statewide student assembly.